This invention relates to a non-linear optical material and a non-linear optical device.
Non-linear optical materials are the materials exhibiting so-called non-linear optical effects, in which non-linear responses result from induced polarization of electrons by the electrical field created by the light incident on the materials. These optical effects are generally due to the second and higher order terms in the following equation of EQU P=.kappa..sup.1 E+.kappa..sup.2 E.multidot.E+.sup.3 E.multidot.E.multidot.E.multidot.+---- +.kappa..sup.n E.multidot.n
wherein P is polarizability of a material, E is intensity of electrical field, and .kappa..sup.n is non-linear optical susceptibility of the n'th order.
It has been known that, due to a phenomenon known as the second harmonic generation (SHG) obtained by the specific utilization of the secondary effect, an incident light is converted into a light wave corresponding to the second harmonic wave and having a frequency twice as high as the frequency of the incident light or the refractive index is changed by voltage, so that the phenomenon is very conveniently utilized for performing various optical processings including conversion of wavelengths, processing of signals and modulation of laser beams, which are extremely advantageous.
Although inorganic crystals, such as KH.sub.2 PO.sub.4 (KDP), LiNbO.sub.3 or NH.sub.4 H.sub.2 PO.sub.4 (ADP), have hitherto been used as the non-linear optical materials, they have disadvantages that single crystals having high optical purities are very expensive, that they are so deliquescent as to be inconvenient in handling, and that the non-linear sensitivities thereof are not so high. On the other hand, since the utility of organic materials as suggested in 1983 in the symposium in the American Chemical Society, organic crystals of urea, aniline base compounds or the like were reported to be utilizable as non-linear optical materials. However, these organic compounds do not exhibit satisfactory non-linear optical effects, or the compounds which exhibit relatively high level of non-linear effect have a disadvantage that they have light absorptive terminal groups that are significantly shifted towards the long wavelength range to thus limit the wavelength range of the light waves which can be processed therethrough.